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Lecture 12 - EXAM VERSION.docx

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University of Ottawa
Michael Jonz

Lecture 12EXAM VERSIONNovember2412154 AM PROTEIN SORTING ERProtein Sorting to the ERERnetwork of branching tubulesMajor site of protein and lipid biosynthesisocaptures selectred proteins from cytosol as they are being synthesizedMembrane seperates ER lumen from cytosol and mediates transfer of proteinsTransmembraneprotein that spans protein at level ER or in plasma membranepartly translocated across the ER membrane and become embedded in it water soluble proteinsnot attached or intergrated in membrane free float in lumen or excreted to outside of cellfully translocated across the ER membrane and are released into the ER lumenCotranslational translocation primarilyimport of proteins begins before polypeptide chain is synthesized completelyProtein is never released into cytosol and never in danger of folding up before reaching translocator in ER membraneMany of the cells proteins must first pass through ERPost translationalimport of proteins into mitochondria chloroplasts nuclei and peroxisomesCahperone proteins are not required to keep protein unfoldedRibosome that is synthesizing the potein is directly attached to ER membrane rough ER Role of Ribosomeprotein complexes that associate with mRNA and catalyze synthesis of proteinRibosome with polypeptide chain will be directed to ER membrane only if the appropriate signal sequence is present or itll stay in the cytosolMembrane bound rough ERoattached to cytosolic side of ER membrane oengaged in synthesis of proteins that are being concurrently translocated to ERFreeounattached to membrnaeosynthesize all other proteins encoded by nuclear genomeomove randomly between segregated populations of mRNA moleculesFigure Synthesize the same protein because synthesizing same RNApool of ribosomes used to synthesize cytosolic he proteins or those transported 1to ER cytosolER signal sequence on polypeptide chain directs the engaged to ER membrane1mRNA molecule remains permanently bound to ER as part of polyribosomeoPolyribosome ribosomes bound to single Mrna moleculeRibsosomes recycled at the end of each round of protein synthesis then subunits 1released in pool
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